Re: M2PA: resynchronization of sequence numbers after PO

sdjugum,

Figure 16 is consistent with the text in section 4.1.4.  Each side shall not
send data messages until it has sent LSR (and then only when other conditions
permit).  It is, however, better to wait until LSR is both sent and received
before starting traffic.  Consider the following:

   LSPO -----------\
   LSPR -----------\\
   LSPO -----------\\\
                    \\\--------> LSPO ---> RPO
                     \\--------> LSPR ---> RPR  ------+
                      \--\                            |
                       /--\----- LSR  <--- Continue <-+
                      //---\---- DATA
                     ///----\--- DATA <--- DATA
    LSR <-----------///      \-> LSPO ---> RPO
   DATA <-----------//
   DATA <-----------/

Which is sending data immediately after sending LSR.  Note that the DATA is
now in the RTB and therefore will be lost on changeover.

Contrast that with the following:

   LSPO -----------\
   LSPR -----------\\
   LSPO -----------\\\
                    \\\--------> LSPO ---> RPO
                     \\--------> LSPR ---> RPR  ------+
                      \--\                            |
                       /--\----- LSR  <--- Continue <-+
                      /    \
                     /      \    (TB) <--- DATA
    LSR <-----------/        \-> LSPO ---> RPO

Which is waiting for received LSR before sending data.  Note that the DATA is
either in Level 3 or in the TB at Level 2 and is therefore not lost on
changeover.

Therefore, waiting for both LSR sent and received reduces message loss and
missequencing.

All that the RFC requires is that DATA not be sent _before_ sending LSR, it
does not place any requirements on how long one waits after sending LSR before
restarting traffic.

--brian

sdjugum wrote:                                   (Fri, 16 Nov 2007 10:29:32)
> 
> Hi Brian,
> 
> Since the question remained open I would just like to repeat it so that it
> is not left unanswered.
> 
> The example metioned below clearly indicates that side B uses BSN received
> in LS Processor Recovered message to synchronize it's sequence numbers and
> that it begins sending messages after it has sent LSR (stream 1). 
> So, the question is - what should it do at reception of LSR (stream 1) from
> peer? Example clearly shows that LSR is ignored and that BSN received in it
> is not used.
> 
> So, which message should be used for synchronization and sending resume at
> side leaving RPO - LS PR or LSR (stream 1)?
> 
> Best regards,
> Sasa
> 

-- 
Brian F. G. Bidulock
[email protected]
http://www.openss7.org/